Additionally, many civilizations' unique units come with special promotions, which most of the time transfer when they upgrade to more advanced units. Sometimes, these are just generic promotions earned as part of the "special" abilities of the unit in question. In this case, if the unit had already acquired the promotion earlier (through earning Experience before the upgrade), it will get nothing on upgrade, so you should plan its earlier development accordingly.

Note that many promotions apply only to certain classes of units - for example, melee, ranged, naval, etc. Also, several key promotions have multiple stages, marked as I, II or III - each subsequent stage of these can only be acquired after you earn the previous stage.

The highest-level promotions require all stages of another lower-level promotion. Note that they are usually available across classes (that is, both for melee and ranged units, for example), but they also require different promotions for the different classes.